Account for gamePly after each move

Rename startPosPly to gamePly and increment/decrement
the variable after each do/undo move. This adds a little
overhead in do_move() but we will need to have the
game ply during the search for the next patches now
under test.

Currently we don't increment gamePly in do_null_move()
becuase it is not needed at the moment. Could change
in the future.

As a nice side effect we can now remove an hack in
startpos_ply_counter().

No functional change.